In some power supplies, the supply voltage Vcc power-off time is fairly long. The rate of decrease from Vcc to 0 depends on the capacitive and inductive load of the power supply. A typical power-off time for a power supply providing a supply voltage to a sound card in a computer system or cellular telephone is about 120 ms to go from 12 to 0.5 volts, for example. A problem associated with such an audio amplifier is a loud popping noise generated by the amplifier as the supply voltage is switched-off.
One approach to reduce or minimize power-off noise will now be discussed with reference to FIG. 1. A Class B audio amplifier 20 is connected to a 12 volt power supply 22. The audio amplifier 20 includes an input for receiving an audio input signal VI and an output for providing an amplified audio output signal VOUT. A speaker 24 is connected to the output of the audio amplifier 20.
The audio amplifier 20 typically includes a supply voltage rejection circuit 28 (FIG. 2) for suppressing noise from the power supply 22. The audio amplifier 20 thus includes an input for receiving a supply voltage rejection signal VSVR for the supply voltage rejection circuit. The capacitors CIN, CP, CS and COUT are external capacitors to the audio amplifier 20. To reduce the power-off noise heard at the speaker 24, transistors Q1 and Q2 are connected to the power supply 22, to the input of the audio amplifier 20 receiving the supply voltage rejection signal VSVR, and to the output of the audio amplifier providing the amplified audio output signal VOUT.
The transistor Q1 includes a base terminal connected to the power supply 22, a collector terminal connected to the base terminal of transistor Q2, and an emitter terminal connected to the input of the audio amplifier 20 receiving the supply voltage rejection signal VSVR. Transistor Q2 includes a collector terminal connected to the output of the amplifier 20, and an emitter terminal connected to a voltage reference, such as ground. When a rate of decrease of the supply voltage VCC is greater than a rate of decrease of the supply voltage rejection signal VSVR, i.e., VCC>VSVR, transistors Q1 and Q2 are turned on. This causes the output of the amplifier 20 to be shorted and the output noise is thus minimized.
However, when the supply voltage VCC does not decrease as fast as the supply voltage rejection signal VSVR, i.e., VSVR>VCC, transistors Q1 and Q2 will not be turned on. The supply voltage rejection circuit 28 of the amplifier 20 is still active. When the supply voltage VCC is larger than VSVR by 1 to 2 times the conducting voltage Vbe for at least one transistor Q3 within the supply voltage rejection circuit 28, transistor Q3 is saturated. Transistor Q3 and other portions of the supply voltage rejection circuit 28 are best illustrated with reference to FIG. 2.
Referring now to FIG. 3a, a graph illustrating a rate of decrease of the supply voltage VCC, the supply voltage rejection signal VSVR, and the audio output signal VOUT at power-off of the power supply 22 is provided. As discussed above, when the supply voltage VCC is larger than VSVR by 1 to 2 times the conducting voltage Vbe for transistor Q3 within the supply voltage rejection circuit 28, transistor Q3 is saturated.
When transistor Q3 is saturated during power-off, ripples present in the power supply 22 are fed into the supply voltage rejection circuit 28 and amplified by transistors Q4 and Q5. As a result, the loud popping noise during power-off can be heard at the output of the amplifier 20 via the speaker 24 connected thereto. FIG. 3b is an expanded graph of the audio output signal VOUT illustrated in FIG. 3a to highlight the noise present during power-off of the power supply 22. An audio amplifier that is not associated with this popping characteristic is thus desirable.